New evidence suggests that caffeine affects women's estrogen levels in different ways

It's no secret that coffee affects our bodies. In fact, that's why most of us drink it—for a quick energy boost in the morning (and afternoon, and night, and really anytime). But a new study published in the American Journal of Clinical Nutrition suggests that the caffeine levels commonly found in a few cups of coffee could also be altering women's estrogen levels, potentially protecting them from—or increasing the risk of—certain hormone-related diseases.

While the study authors note that the amount of caffeine in about two cups of coffee likely won't alter a woman's ability to ovulate in the short term, the results do suggest the caffeine-fueled changes in estrogen could affect a woman's risk of developing certain diseases down the line. That impacts a lot of women, considering that nearly 90 percent of women ages 18 to 34 drink the caffeine equivalent of 1½ to 2 cups of coffee a day.

In the study of 269 women, researchers took measurements of hormone levels across two menstrual cycles, while repeatedly recording how many caffeinated drinks the women were consuming. Interestingly, they discovered that caffeine affects estrogen levels differently among women of different races. Moderate daily caffeine intake—about two cups of coffee or 200 mg a day—lowered estrogen levels among white women, potentially offering protection from diseases linked to higher estrogen, such as reproductive cancers. On the other hand, Black and Asian women who drank the same amount of caffeine experienced elevated estrogen levels.

So what gives? The difference in how caffeine changes hormone levels among different races could be tied to different rates of metabolizing caffeine. "While further research is needed, moderate consumption of caffeine most likely will not increase the risk of breast cancer among white women, and may even protect them from developing it," says lead study author Karen Schliep PhD, MSPH, research assistant at the University of Utah. As for other races, such as Asians and blacks, more research is needed before conclusions can be made.

Bottom line, says Sharima Rasanayagam, PhD, director of science at the Breast Cancer Fund, “this study illustrates that we can't assume that one size fits all when exploring hormonal factors—we need to pay attention to race, ethnicity, and socio-cultural issues." Plus, since the majority of the women in the study also drank soda on a daily basis, researchers plan on investigating what components in soda, such as added sugars, may contribute to the elevated estrogen levels.